1. Field of the Invention.
The invention relates generally to a direct access storage device (DASD) of the type utilizing partial-response maximum-likelihood (PRML) detection, and more particularly to a method and apparatus for (n+0.5)T acquisition timing loop algorithm for PRML data detection,
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use, Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.
Data channels in DASD units typically utilize a synchronous detection scheme. Both peak-detect and PRML are examples of synchronous data detection channels where synchronous refers to the frequency and phase locking of the channel to the readback signal in order to detect the data properly. In a DASD, typically a preamble pattern is written before the start of any data block. The preamble pattern often referred to as a sync field is utilized by the channel to acquire initial gain, frequency and phase lock to the readback data stream. The sync field and following data block are written together using a single continuous write operation. During readback the channel acquires timing lock to the sync field so that the synchronous detection of the data block following the sync field is guaranteed.
A timing loop problem results when a programmable digital filter is provided as part of the PRML channel. The tracking timing loop operates from the digital filter output, while the acquisition timing loop bypasses the digital filter and operates directly from samples from an analog to digital converter (ADC). In order to avoid undesirable phase shift when switching from acquisition timing mode to tracking timing mode, certain constraints are forced on the tap weights of the digital filter. A need exists for an acquisition timing loop method and apparatus that provides additional freedom for the selection of digital filter tap weights and that is simple to implement.